That sucks you can't figure out what it is and damn thats expensive! I just spend $245 on an adult pastel girl (cost me $3000 a few years back!) that had some sort of infection and my cytology was only $50! My bill would have been much more but luckily the head of the vet school took a special interest in her and did a couple of free tests for me! She's doing great now!
Good luck on your snake and hope you figure out whats going on!